A growing demand for video/data transmitted the Internet and other networks increases need for higher speeds in transmission and delivery systems, especially at what is known as “the last mile”, i.e. the last part of the transmission line reaching a user. Nowadays, telecommunications operators are deploying a large number of fibre links in order to deliver broadband access to the user based on Passive Optical Networks (PONs). With the increasing number of fibres and cables, the probability of fault at some point of the PON also increases, generating a demand for fibre inspection.
The need for optical fibre maintenance functions for prevention and fault maintenance has been extensively discussed, for instance by International Telecommunication Union's Telecommunication Standardization Sector Study Group 6 (ITU-T SG6). Most studies considered by the group were based on the use of traditional Optical Time Domain Reflectometry (OTDR) in order to localize faults with reasonable accuracy without in optical networks without interfering with data transmission.
It is desirable to implement an inspection solution that neither interferes with data transmission, nor requires high additional capital and operational expenses. Furthermore, the implementation of Subcarrier Multiplexed (SCM) signals in PONs enables the use of a subcarrier for fibre monitoring purposes, rather than monitoring faults over a separate optical carrier, which would require a separate transmission and detection scheme.
For instance, methods are known that employ a pulse monitoring scheme modulated into the SCM signal in similar fashion to an OTDR solution, without the need for additional transmission equipment. Aside from the need for pulse averaging in order to obtain sufficient resolution, this technique also requires additional filtering to separate the monitoring pulse from the subcarrier signal. Furthermore, the monitoring pulse and its harmonics may interfere with data transmission.